Use of gratings for line narrowing of lasers is well known. See, for example, U.S. Pat. Nos. 5,095,492 and 5,852,627. Typically, a line narrowing unit consists of a grating and a beam expander. Such prior art techniques are shown in FIG. 1A which is also FIG. 1 is U.S. Pat. No. 5,095,492 and FIG. 1B which is also FIG. 1 in U.S. Pat. No. 5,852,627. In these prior art embodiments, the laser is tuned by pivoting the pivot mirror as specifically shown in FIG. 1B to change the angle of illumination of the grating which in each of these cases is arranged in Littrow configuration. The laser could also be tuned by pivoting the grating.
As explained in U.S. Pat. No. 5,095,492 the wave front of the light illuminating the grating typically deviates from a flat front due to distortions produced by the various optical components of the laser. U.S. Pat. No. 5,095,492 discloses a technique for bending the grating to make the lined surface increasingly curved to compensate for the curvature of the illuminating beam. The bandwidth of the laser can be substantially reduced by proper adjustment of the curvature of the lined surface of the grating.